The present invention relates to a plasma reactor with a reactor housing designed as resonator to which is attached a high-frequency coaxial line connected with a high-frequency generator for irradiation of microwaves and in which is located a holder for a substrate to be coated, a vacuum-tight window of microwave-permeable material being positioned in the area of transition from the high-frequency coaxial line to the reactor housing and the reactor housing exhibiting connections for supplying and removing a process gas.
A similar plasma reactor is already known from U.S. Pat. No. 5,501,740. In this unit, microwave energy is supplied via a coaxial line underneath a microwave electrode also serving as a substrate holder, the inner conductor of the coaxial line being connected to the bottom side of the substrate holder. The plate-shaped electrode is connected with the reactor housing by a dielectric ring forming the microwave window.
Sealing of the reactor housing toward the outside or to the coaxial line also occurs via the microwave window. Compared to the external atmospheric pressure, reduced interior pressure prevails in the reactor housing so that relatively high forces act on the substrate holder which tensionally stress the inner conductor of the coaxial line connected with the substrate holder. Accordingly, stable attachment and external anchoring of the inner conductor and correspondingly complicated mechanical securing measures are required.
Further disadvantageous is the fact that in this design, the substrate holder is only very poorly accessible so that it is difficult and complicated to realize techniques such as, for example, substrate cooling, substrate rotation or positioning, temperature control, etc. In particular, supply lines need to be led here in disadvantageous fashion through the inner conductor of the coaxial line. However, only very limited room is available here.
Known from U.S. Pat. No. 4,866,346 as well as DE 195 07 077 are plasma reactors in which the substrate holder has relatively good accessibility.
In the case of U.S. Pat. No. 4,866,346, however, there is the disadvantage that the position and form of the plasma depend very sensitively on process parameters such as gas pressure and coupled microwave power. In addition, these two process parameters are not selectable independently of each other, but rather are adjustable in mutual dependence only within a relatively narrow correlation range. Upon leaving this correlation range, a sudden spatial shift of the plasma often occurs, which, in addition to disturbing the deposition process, quickly destroys, for example, a quartz glass limiting the reaction volume. A further disadvantage is the strong dependence of the position of the plasma on the geometric limit conditions and the arrangement of the reaction unit.
With DE 195 07 077, very good plasma form and stability can be attained, to be sure; however, a deposition chamber positioned within the reactor housing and surrounded by a quartz bell is provided here, in which the substrate holder is located. This leads to unfavorable size ratios since the resonator is clearly larger than the actual deposition chamber.
Known from EP 778 608 A2 is a plasma generator and a process for producing a plasma whereby through several waveguides microwave energy is carried to a ring-shaped distribution channel. The irradiation in the reactor chamber follows through several, around the perimeter arranged microwave windows. On the path of the distribution channel a damping exists created through enclosure position differences in coupling performance, which stimulates a non-rotationally symmetric mode, from which an non-symmetrical and non-uniform plasma results, so that in the coating of a substrate in plasma, no homogenous coating is obtainable.
It is the object of the present invention to provide a plasma reactor of the type mentioned above in which the substrate is accessible in good and simple fashion, among other things, for cooling measures, for positioning, temperature control, and similar measures. In addition, the produced plasma should be stable with regard to plasma position, intensive, spatially extended, and homogeneous. Finally, the plasma reactor should exhibit an overall compact design.
In solving this problem, it is proposed that the high-frequency coaxial line, at its delivery end outside the reactor housing, is funnel-shaped and essentially rotationally symmetric and directed at the periphery of the reactor housing toward the essentially ring-shaped, microwave window for a coupling that is distributed over the entire ring surface of the microwave window.
As a result of the microwave irradiation occurring outside at the periphery of the reactor housing, practically the entire interior volume of the reactor housing is available as interior space for the actual deposition chamber. Accordingly, particularly favorable size relationships result for the reactor, with the advantage of a compact design. In addition, practically the entire outer wall of the reactor housing and thus the deposition chamber can be easily cooled since it is highly accessible. Thus, in particularly simple fashion, the possibility exists for the reactor walls to consist extensively of double-wall, metallic, water-cooled walls.
The ring-shaped wave-guide section in connection with the microwave window positioned peripherally on the reactor housing also has the advantage that the coupling field in the peripheral direction overalll has the same phase, which results in a rotationally symmetrical field and plasma distribution. Also, the coupling is distributed over a large surface. As a result, high microwave power levels can be coupled without high electric field intensities developing at the microwave window and thus without the danger of window discharge. This is an important condition especially for the stability of the plasma position.
Especially advantageous here, too, is the fact that the operating ranges of microwave irradiation, on the one hand, and substrate holder with associated elements, on the other, form separate operating ranges which can thus be designed and dimensioned corresponding to current requirements. In particular, design optimizations can be independently undertaken as a result.
The separate operating ranges also give good accessibility of the substrate holder and thus especially simply designed supply lines (for example, for coolants) and means for positioning (e.g., substrate rotation) as well as for temperature control and the like.
An advantageous embodiment of the present invention provides that the reactor housing is subdivided into two sections electrically insulated from each other, at least one of these sections being combined with a substrate holder while the other section electrically insulated therefrom serves preferably as the electrode for d.c.-voltage or high-frequency discharge.
The peripherally extending, ring-shape constructed microwave window is formed advantageously of an electrically insulating, microwave permeable material, preferably quartz, and is inserted between the two housing sections as an insulator.
In this case, the microwave window, itself, forms the insulator between the housing sections so that an additional insulator can be foregone and construction is simplified. The use of quartz for the microwave window has the advantage that such a window also withstands very high temperature gradients.
If necessary, the connection for supplying or removing a process gas can face the substrate holder and can preferably be directed approximately centrally toward the substrate holder. This central gas supply favors homogenous coating as well as high rotational symmetry in the deposition process.
A modified embodiment of the present invention provides that a substrate holder is preferably combined with each of the two housing sections and that these substrate holders are positioned particularly directly opposite to each other and preferably both in a common area of high field strength.
As a result, simultaneous coating of two substrates and thus energy-saving economic processing are possible.
If necessary, the resonator formed by the reactor housing can also be designed such that there exist spatially separated areas of high or maximum field strength in which the substrate holders are positioned.
Additional embodiments of the invention are presented in the further subclaims.